Summary of work: Family history studies showed a familial aggregation of Alzheimer's disease among first-degree relatives of Alzheimer's disease probands compared with controls. A case report shows that dementia in Down's syndrome may occur without mental retardation. DNA repair studies show defective nucleotide excision repair of free-radical induced bulky lesion in Down's syndrome and sporadic and familial Alzheimer's disease. Apolipoprotein E4 allele in Down's syndrome increases the age-related incidence of dementia and shortens life expectancy.